2.79b Blender texture paint stroke oddity
Suggestion is to have a fresh start.
Delete the Brush in question and create a new one.  Use the new Brush and see if the problem persists.

You may have changed a setting unintentionally. There are many settings in Blender that you or I or some others may not have the time to track down at the moment. In the sense of triage you may have more important things to occupy your concerns.